High-temperature storage and thermal cycling studies of thick film and wirewound resistors

The operating ambient temperature for underhood automotive and aerospace applications is increasing. This work was undertaken to evaluate the suitability of thick film and wirewound resistors for distributed aircraft control systems in a 200 degreesC-225 degreesC operating environment. High temperature stability testing of power wirewound and thick film resistors is reported. Dale power wirewound 1 Omega, 100 Omega, and 10 kOmega resistors with power ratings of 5 W and 25 W were tested. The TCR of the 100 Omega, and 10 kOmega resistors was very small, however, the 1 Omega resistor varied by 5% over the temperature range from 25 degreesC to 300 degreesC. Stability with long term storage (10 000 h) at 300 degreesC was measured for the wirewound resistors unpowered and powered at 20% of rated power. With the exception of the 10 kOmega/25W resistor, the change in resistance was less than 4%. Wirewound resistors were also thermal cycled 1000 times over a temperature range from -55 degreesC to 225 degreesC with only one failure due to a broken internal connection. Three 900 Series thick film resistor pastes from Heraeus-Cermalloy were studied: 100 Omega/sq., 1 kOmega/sq., 10 kOmega/sq. The temperature coefficient of resistance (TCR) was measured from 27 degreesC to 500 1 C in 50 degreesC increments. The change in resistance was <+/-6% up to 300 degreesC. A 2 x 2 matrix of variables was included in the 300 degreesC storage test: untrimmed resistors, resistors trimmed up 50% in value, unpowered, and powered at 1/8 W. Palladium/Silver was the initial termination choice for these 300 degreesC studies, but silver migration under electrical bias lead to electrical shorts between conductor traces on the substrates with powered resistors. Gold terminated thick film resistors were used for powered storage testing at 300 degreesC. The change in resistance after 10000 h at 300 degreesC was <+/-3% for all test combinations.